Cross-polarization, nuclear magnetic resonance spectroscopy experiments are performed by spinning a sample material in an applied magnetic field. As a result of the interaction between the sample and the magnetic field, the sample emits electromagnetic signals at characteristic resonant frequencies. The resonant frequency depends on the strength of the magnetic field, and the nuclear and structural properties of the test sample. The signals so produced are collected and processed using spectrum analysis instrumentation to obtain a plot of signal intensity versus frequency, i.e., an intensity-frequency spectrum.
Resolution is the process of distinguishing between discrete signals produced from a single sample. The maximum resolution, i.e., greatest distinction between signals, is achieved, in an exemplary amendment, by spinning a test sample several thousand times a second at a 54.7.degree. angle with respect to the magnetic field. This angle is known as the "magic angle."
The usual method of attaining a reference magic angle is to observe bromine-79 spectral signals emitted from a spinning sample of potassium bromide. When a characteristic peak signal of the sample is observed, the angular position with respect to the magnetic field is noted. This angular position must be accurately reproduced for subsequent test samples. Since overlap of many spectral lines can occur at small angular variances from the magic angle, accurate reproducibility is a critical factor.
Magic angle calibration is conventionally performed by either reproducing the angle mechanically or mixing a test sample with potassium bromide and observing bromine-79 signals to reset the angle. However, both of these approaches have certain disadvantages which will be described below.
Mechanical calibration systems generally use a mechanical "stop" to reproduce a reference angle. In particular, after observing a bromine-79 reference signal, the spectroscopist sets a mechanical stop which is attached to the angular adjustment mechanism of the sample holder. The stop prevents the sample from being positioned past the magic angle. In order to test a subsequent sample, the holder is rotated to a 90.degree. position, the reference material is replaced by a test sample, and the holder is rotated back to the mechanical limit of the stop.
One serious disadvantage of the mechanical stop calibration procedure is a lack of accuracy. This approach does not guarantee angular reproducibility of .+-.0.1.degree., which is the measure of accuracy required to prevent serious signal degradation for certain test samples. Moreover, mechanical adjustment and calibration devices commonly experience problems such as gear backlash, hysteresis and long-term slippage or wandering of set positions. These problems cause variations in the angular position of a sample each time the holder is repositioned, and such variations significantly degrade reproducibility.
As noted above, an alternative calibration procedure is to mix potassium bromide with the test sample and set the magic angle by observing bromine-79 spectral lines. However, while this approach ensures accurate attainment of the magic angle for each experiment, the procedure reduces the volume of the tested sample which, in turn, extends the time required to perform the experiment. An additional disadvantage is that some test samples should not be mixed with foreign materials, such as potassium bromide.
A common disadvantage of both procedures described above is that neither alternative allows small angular deviations from the magic angle to be accurately reproduced and also completed within a short time frame. Small angular deviational experiments are typically performed to elicit additional spectral information from a test sample.